... It appears that speaker/listening room placement is the only way to minimize the effect of nulls?
Correct, a null and a peak are destructive interference and a constructive interference respectively.
However the room dimensions will define the peaks that exist in the corners and parallel walls.
This is why I mentioned a microphone in post #2 and #4. One can calculate what they should be, and where. One can also measure what the peaks are at a variety of “where” positions. Knowing that, then you have a map of where the modes are high that need to be absorbed, and what freqs they are at.
We do not get pressure nulls in the corners as easily as we get them in the listening position(s). (We get velocity nulls in the corners and walls,) And your treatments will be located at the walls, so you have the problem constrained there at the wall, and only need to measure there. (But measurements at the listening position should likely also be done.)
As the long modes will be longer, they will be at a lower frequency… and at a lower freq than say the short wall distance modes. And if the traps are low-Q, then they will largely suck out what they should. if they can be made with a lower Q and tuned in-situ then it get easier to be closer to perfection, but the low-Q obviates the need for being “overly perfect”.
Treatments fix the peaks. And yes placement the nulls.
So focus on the peaks for the wall treatment.